The AC (Acromio-Clavicular) Joint is a small joint at the top of the shoulder formed between the outer end of the collar bone (clavicle) and the acromion (shoulder tip) which is part of the shoulder blade that ‘leans’ forward over the top of the shoulder. There is much less movement in the AC joint compared to the main shoulder joint (glenohumeral joint) with movement only really occuring when the arm is lifted above shoulder height or across the chest. Within the joint there is a pad of cartilage called the meniscus (like in the knee joint) which helps to cushion and move the joint. By the time the patient is in their 40s this cartilage pad is often not functional and has degenerated. The capsule of the joint helps to keep it stable and the surrounding ligaments, particularly the superior acromioclavicular ligament, help to reinforce this stability.

AC Joint Arthritis
This often co-exists with Impingement Syndrome but can exist in isolation and there is little evidence to suggest the two conditions are linked. As you get older the AC joint narrows considerably, this is most apparent on an x-ray and over 50% of patients over 65 will have x-ray evidence of AC joint arthritis. However it is responsible for symptoms in a much smaller number of patients than the x-ray findings might suggest.
Symptoms
Pain localised to the AC joint generally in patients over 60.
Pain worse on overhead activities, lifting, sleeping on the affected side with the patient often being woken at night when turning onto the affected side
Pain worse on the arm being taken across the chest eg to wash other arm etc.
Pain worse going behind the back eg doing bra up, getting something out of back pocket.

Diagnosis

Tenderness localised to the AC Joint on palpation (pressure applied over the AC joint)
Pain in AC joint when lifting the arm above shoulder height
Pain on cross arm adduction (arm lifted across chest to touch other shoulder). This is often called the Scarf test.
X-ray - narrowing of AC joint associated with other changes of arthritis such as osteophyte formation and erosion of the smooth joint surface.

Treatment

Anti-inflammatory (NSAIDs) medications and pain-killers are usually recommended to reduce the inflammation and help the pain

Physiotherapy; may help some of the symptoms but is much less effective than in other shoulder conditions

AC Joint Injection: a local anaesthetic and steroid injection is often beneficial to reduce pain. The injections will last from a few weeks to a few months but rarely resolve the pain long term

If the above treatments are ineffective or only provide short term relief then surgery in the form of an Arthroscopic AC Joint excision may be needed (See Arthroscopic AC Joint Excision)

AC Joint Problems in Younger Patients
The AC joint is maximally loaded with heavy overhead activities. Therefore in either athletes (Powerlifters, Gymnasts, Collision Sports) or in heavy manual labourers especially if overhead (eg plasterers) then the AC joint may be overloaded leading either to damage to the meniscus within the AC joint or osteolysis of the outer end of the collar bone (clavicle). Osteolysis is rare but is essentially a stress fracture of the outer end of the clavicle where the bone has been repeatedly overloaded exceeding the ability of the bone to heal sufficiently between ‘insults’.

Symptoms

Pain localised to the AC joint in patients under 40

Pain worse on overhead activities, lifting, sleeping on the affected side

Pain worse on arm crossing the chest eg to wash other arm etc.

Diagnosis

Tenderness localised to the AC Joint on palpation (pressure applied over joint)

Pain in AC joint when lifting the arm above shoulder height.

Pain on cross arm abduction (arm lifted across chest to touch other shoulder). This is often called the Scarf test

X-Ray: loss of bone from outer end of clavicle on x-ray is seen in osteolysis

MRI: swelling and inflammation around AC joint

Treatment

Rest: 6-8 weeks complete rest from overhead weightlifting is required to allow the bone to heal.

Anti-inflammatory (NSAIDs) medications and pain-killers are usually recommended to reduce the inflammation. However, NSAIDs can sometimes reduce bone healing so need to be used with caution depending on the x-ray appearance.

Physiotherapy can be helpful for local pain-relief and to balance the muscles which support the AC joint

AC Joint Injection: a local anaesthetic and steroid injection is often beneficial to reduce pain. The injections will last from a few weeks to a few months and they are helpful as they reduce the pain and when combined with rest will allow the swelling and inflammation to settle and the bone to heal in the case of osteolysis.

If the treatments listed above do not settle the pain then keyhole surgery to removed the damaged meniscus, AC joint and eroded bone (outer end of the clavicle) may be needed (see Arthroscopic AC Joint Excision)